The present invention relates generally to sensor technology. More particularly, the present invention relates to wireless devices used to record the conditions of many items (e.g., thermal protection system tiles on a space vehicle).
In many sensing applications, it is desirable to determine whether an event has occurred. This event may include an over-limit such as surpassing a property threshold (i.e. a detrimental concentration of a bacteria in food) or may include a time integrated exposure, a material phase change or combination thereof which may form a history. In many applications, monitoring of the event as the event occurs may not be possible or practical, and thus it may be desirable to determine whether the event has occurred subsequent to occurrence. In addition, many applications require that many items be separately analyzed to determine whether the event has occurred for any of the items. Further, the remote nature of many sensor applications, such as a sensor placed within a sealed container, may demand a wireless form of communication that permits non-invasive interrogation.
The majority of current wireless monitoring systems provide real time response which may not be suitable or necessary in applications where a well defined event is being detected. For the case of detecting an infrequent event, continual real time information feedback for a large number of items may be inefficient. In addition, the complexity and size of real time sensors may make application in many environments unpractical.
Many sensing applications provide significant challenges due to inaccessible, environmentally prohibitive, or functionally disadvantageous conditions. Current device designs often cannot meet this need. Active sensors have been combined with various forms of wireless data communication, but these devices are generally large and require a battery or other subsequent power source. Further, the battery power source and/or attendant wiring may have a limited range of operation, thereby making the system fragile.
One particular example of the need for multiple event-recording devices for a large number of items is in developing faster methods for inspecting and maintaining the structural and functional integrity of reusable launch vehicles (RLVs), such as the Space Shuttle. One type of primary failure mode that can affect the Space Shuttle thermal protection system. (TPS) tile performance is thermal breach. Thermal breach may be caused by hot gas penetration and over-temperature conditions at the TPS bond line during earth reentry, and results in the loss of gap fillers and/or the dimensional instability of the TPS. Thermal breach is difficult to detect since thermal penetration may damage the interior surface and/or the TPS bond line without clearly showing external indication of damage on the tile""s top surface.
Current shuttle inspection techniques involve visual and manual inspection of each of the gaps between all of the nearly 22,000 tiles using a hand held filler gauge to measure the thickness and depth of spaces between the tiles. The inspection may further include looking for other effects of thermal stress such as surface damage, discoloration, silicon deposits, or texture changes of the TPS coating. Thermal protection tiles are bonded to a vehicle using an organic adhesive. If the organic adhesive (normally a shiny red) appears dull or black, a closer inspection is required to determine the extent of charring. Presently, Space Shuttle recertification for reflight requires tens of thousands of person hours to manually inspect each of the 22,000 shuttle tiles. The substantial cost of TPS inspection ranks second in operations costs only behind the propulsion system.
Not only is the current approach very slow and expensive, but human inspection is inherently error-prone. Repetitive inspection of the thousands of tiles leads to inspector fatigue and greater potential for error. The scaffolding required to inspect the vehicle is additionally costly and time consuming to set up. For the next generation of reusable launch vehicles (RLVs), it is desired to reduce turnaround time to 24 hours. As current detection methods are prohibitively time consuming and expensive, an automated means of post reentry inspection of the TPS is desirable.
One proposed approach to maintaining RLV systems involves the use of discreet active sensors which rely on a power source directly connected to the sensor. Examples of active sensors which have been used to discretely monitor RLV systems such as propulsion and guidance include strain gauges, thermocouples, and fiberoptic sensors. However, the size and complexity of the active sensors do not allow for monitoring of the TPS since the abundant number of tiles would necessitate a prohibitively excessive amount of weight and wiring.
In view of the foregoing, there are desired improved structures and techniques for wireless threshold sensing and recording for multiple objects.
The present invention provides a device that can be interrogated to determine its identity and its state. The state indicates whether a particular physical or chemical event has taken place. In effect, the device records the physical or chemical event. The identity of the device allows it to be distinguished from a number of similar devices. Thus this invention finds particular usefulness in the context of an array of devices that can be probed by a wireless interrogation unit. The device tells the interrogator who it is and what state it is in. The interrogator can thus easily identify particular items in an array that have reached a particular condition.
One aspect of the invention provides a device for reporting a physical or chemical event or state (possibly a time-integrated condition). The device may be characterized as including the following elements: (a) a sensor for detecting the physical or chemical event or state without using a power source; (b) a recording mechanism integral with or coupled to the sensor for recording that the physical or chemical event or state has occurred; (c) a tag that contains identification information that can distinguish the device from a plurality of similar devices; and (d) a transponder, coupled to the recording mechanism and the tag, configured to transmit a signal indicating the physical or chemical event or state and the identification information when triggered by a wireless interrogation signal.
In one specific example, the sensor is a temperature sensor and the physical or chemical event or state is exceeding a threshold temperature. A suitable device for this purpose may include a circuit as the recording mechanism and a fuse in the circuit as the sensor. When the threshold temperature is exceeded, the fuse opens at least one path through the circuit, thus changing the state of the recording mechanism. In one embodiment, opening the path changes the resonance frequency of the circuit. One way this can be detected is by probing the device with a swept- or stepped-frequency interrogation signal and detecting the peak frequency of the signal sent from the transponder.
To keep the device small and simple, it is preferably passive; that is, it does not include its own power source. Thus, the transponder component is preferably passive. In the example described, the radio frequency interrogation signal (or a separate energizing signal) may provide the transponder power. The sensor component and/or the recording mechanism are also preferably passive. In some cases, the physical or chemical event or state itself provides the power for the recording mechanism to record the event. For example, exceeding a threshold temperature melts a fuse in the above example.
Generally, the recording mechanism assumes a first state when the physical or chemical event or state has not been recorded and assumes a second state when the event or state has been recorded. Either state can be reported by the transponder in response to a wireless probe. Preferably, the recording mechanism can be reset after interrogation but cannot spontaneously return from the second state to the first state when the physical or chemical event or state ceases. Thus, when the temperature drops back below a threshold temperature (the physical event ceases), the recording mechanism will retain information that the device once exceeded the threshold.
The recording mechanism can take many different forms. It may be a mechanical structure, such as a latching structure, a microelectomechanical device, an integrated circuit memory device, an electrical circuit, an optical circuit, and the like. Likewise, the signal transmitted by the transponder may be provided on many different carriers. For example, the signal may be provided on an acoustic wave, a radio frequency wave, an electrical field, a magnetic field, a microwave frequency wave, a light wave, and the like. If the carrier is a wave of appropriate frequency, the transponder may include a modulator and an antenna.
To allow concurrent detection of multiple events or to allow more precise detection of a parameter value (e.g., temperature), a device of this invention may be designed with a plurality of recording mechanisms or sensors, each configured to record or detect a different physical or chemical event or state. The different physical or chemical events or states may be associated with different physical properties (parameters) such as temperature and strain. Alternatively, the different physical or chemical events or states may represent different thresholds of a single physical property. For example, the different thresholds of the single property may be two substantially different temperature thresholds. In this way, a maximum or minimum temperature may be precisely bracketed as determined by which of the sensors or recording mechanisms changed states.
In some embodiments, a single structure serves multiple purposes. For example, a resonant electrical circuit may serve as an antenna, a modulator, and a recording mechanism.
Another aspect of the invention provides a method for recording and reporting a physical or chemical event or state pertaining to a parameter being monitored using a wireless event recording device of the type described above, for example. The method includes passively recording the event using a recording mechanism included in the wireless event-recording device. The method further includes externally probing the wireless event-recording device using a wireless interrogator. The method also includes reporting a status of the recording mechanism and an identification of the individual wireless event-recording sensor.
Another aspect of the invention provides a collection of sensor (e.g., an array) including a plurality of spatially separated wireless event-recording devices each being distinguishable from the remaining wireless event-recording devices of the collection when probed by an interrogation signal. The individual devices may be identical to or similar to the devices described above. Thus, each wireless event-recording device may include a sensor for detecting the physical or chemical event or state, a recording mechanism for recording the event, and a transponder, coupled to the recording mechanism. Each of the individual devices is configured to transmit a signal indicating whether the event has been recorded when triggered by the interrogation signal. In a preferred embodiment, the plurality of event-recording devices are mounted at separate locations on a single structure.
Still another aspect of the invention relates to a wireless event sensing apparatus for recording and reporting an event pertaining to a physical parameter in a system being monitored. The apparatus includes one or more wireless event-recording devices in the system being monitored. The apparatus also includes an interrogator for externally probing the status of at least one wireless event-recording device subsequent to the event. A wireless signal from the interrogator may power the transponder of the wireless event-recording device. The interrogator then receives information on the state of the recording mechanism and the identification tag.
These and other features and advantages of the present invention will be described in the following description of the invention and associated figures.